Question

What is green and croaks like a frog?

Answer 1

I live in a country in Europe with only 11 species of frogs and toads. Like in birds, each species croaks / sings with its own specific sound, which is easily distinguishable from that of other species. 
And even among those few species, there is a very wide range of different calling/croaking behaviours. 
Some species, like the agile frog (Rana dalmatina) and the garlic toad (Pelobates fuscus), croak underwater and so weakly that it can only be heard at distances of up to 10 m, at best. So, obviously, the croaking can only attract females if they are already in the water. At the other end of the spectrum, in the European tree frog (Hyla arborea), even a single calling male may be heard (by the human ear, and presumably also by other frogs) at distances of up to 3 km, under favourable conditions. 
In the common toad, Bufo bufo, the males swim around in the water among each other; they do not call for the females, but make weak croaking sounds every time they bump into another male; this croaking means “I´m not a female, don´t try to mate with me”. If a male bumps into a toad that does not croak, then that other toad must be a female, and it will try to mate with it. However, if in a pond there is only a single male toad, then you may hear it call with a sound that is considerably louder and may be heard at least 50 m away - it is obviously trying to attract other toads, especially females. 
The green toad (Bufotes viridis) calls much louder than the common toad. If a male toad has found a nice new pond, it may call very loudly - it may be heard 1 km away, or more; obviously the purpose is to attract other toads. If it is in the same old pond as every year, it does not call as loudly, but still it may be heard several 100 m away. I once observed an unusually large population of that species, with about 10,000 individuals at the breeding site. There were females everywhere, so it had no purpose for the males to call, and indeed, they did not. 
Also in the firebellied toad (Bombina bombina), if a few males have found a new attractive pond, they may call very loudly, and attract other males from distances of at least 1 km. But if they call in the usual old place, where all the other males also are, the calling is weaker and audible only at a few hundred meters distance. 
Many species also have weaker calls telling other males to keep a proper distance. 
The green frog (Pelophylax esculentus) and its relatives are special in that they have a wider repertoire of calling sounds. There are loud territorial calls, advertising to other males that “this is my position in the pond; keep some distance”; there are also other calls of unknown significance, and then there are mating calls, which become increasingly intense in the days just prior to the spawning of eggs; these mating calls are more species specific than the other calls and seem to have the function to stimulate the maturing of eggs in the females´ ovaries. 
So there is a very wide variation in calling behaviour; but no species of frog or toad is mute - in contrast to the newts that have very different mating behaviours.

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